The present invention relates, in general, to steam generator pressure vessels and, in particular, to an apparatus for supplying relatively cool feedwater to a heated pressure vessel while moderating the thermal gradients therein and in the vessel.
The present invention is particularly suitable for the type of steam generators that are associated with nuclear power plants. In this regard, such steam generators may be viewed as comprising a vertically oriented and substantially closed vessel within which a primary fluid which has been heated by circulation through the reactor core and a vaporizable fluid, in the form of feedwater, are made to flow in indirect heat exchange relationship, such that heat is transferred from the heated fluid to the feedwater. Moreover, in accordance with conventional practice, the steam generator vessel contains a bundle of heat exchange tubes with the ends of each of the heat exchange tubes being suitably retained within a pair of tube sheets. The steam generator vessel is generally substantially cylindrical in configuration, and has a tube sheet suitably mounted therewithin, such as to be positioned adjacent but spaced from each of the ends of the steam generator vessel. Each of the heat exchange tubes in the bundle is in turn suitably supported from the steam generator vessel so as to extend longitudinally therewithin, with the respective ends thereof emplaced in a corresponding one of the aforesaid pair of tube sheets. A cylindrical baffle or shroud is disposed about the bundle of heat exchange tubes to divide the steam generator vessel interior into an annular down flow passageway and an axially disposed evaporator chamber containing the bundle of heat exchange tubes. A plurality of feedwater inlet nozzles communicates with the annular down flow passageway. The feedwater inlet nozzles are generally formed as an integral part of the steam generator vessel, and are spaced at a common elevation around the steam generator vessel.
The heated primary fluid enters the steam generator vessel through a primary fluid inlet and is made to flow through the heat exchange tubes of the bundle, and thence discharged out of the steam generator vessel through a primary fluid outlet, to be conveyed through the remainder of the reactor coolant system. The feedwater is introduced through the feedwater inlet nozzles, and is made to flow down the annular passageway until the tube sheet near the bottom of the annular passageway causes the feedwater to reverse direction, passing in heat transfer relationship with the outside of the heat exchange tubes while flowing upwardly through the inside of the shroud. While the feedwater is circulating in heat transfer relationship with the heat exchange tubes of the bundle, heat is transferred from the heated primary fluid in the tubes to the feedwater surrounding the tubes causing a portion of the feedwater to be converted to steam. The steam then rises and is discharged from the steam generator vessel through one or more steam outlets for circulation through typical generating equipment to produce electricity in a manner well known in the art.
The feedwater inlet nozzle is fed by a supply conduit which is connected thereto for discharge into a thermal sleeve that extends within and through the feedwater inlet nozzle and has one end generally formed with or connected to a sparger, the latter distributes the feedwater downwardly through the annular passageway. The thermal sleeve acts as a shield to reduce the temperature gradients between the relatively cool feedwater flowing therethrough, as compared to the heated feedwater inlet nozzle and steam generator vessel.
The relatively large temperature gradients extending through the feedwater inlet nozzle from the warm steam generator vessel to the relatively cool feedwater tend to produce thermal stresses. Thermal gradients, and the thermal stresses resulting therefrom, are particularly aggravated as a result of changes in the feedwater flow through the inlet nozzle of this type steam generator, under certain operating conditions such as during the reactor start-up as well as during changes in the reactor power output. It is during these changes in feedwater flow that there occurs thermal cycling of the feedwater inlet nozzle and the thermal sleeve. Such thermal cycling may induce fatigue failure in the dissimilar metal weld which fixedly secures the thermal sleeve, through a transition ring, to the feedwater inlet nozzle. In fact, due to restricted access to this thermal sleeve weld region, it is difficult to detect and eliminate weld flaws. Moreover, since the nozzle is usually made of low alloy steel, it corrodes much faster than the thermal sleeve which is made of corrosion-resistant material. Thus, the feedwater inlet nozzle side of this dissimilar metal weld will be severely thinned. Obviously, this corrosion problem could be eliminated if the feedwater inlet nozzle were made of the same expensive corrosion-resistant material as that of the thermal sleeve. However, the material cost of such a modification would be high because of the heavy section size of the feedwater inlet nozzle. When the cantilever thermal sleeve and sparger unit is subjected to a bending moment by feedwater injection and pressure difference or the occurrence of an earthquake, significant bending and axial stresses will occur at the thinned cross section on the feedwater inlet nozzle side of the dissimilar metal weld. As a result, the thermal sleeve may develop fatigue cracks, and the ensuing leaks of feedwater may flow around the outer surface of the thermal sleeve, and come in direct contact with the feedwater inlet nozzle and hence cause undesirable cooling which may lead to thermal stresses in the area of the feedwater inlet nozzle and the surrounding wall portion of the steam generator vessel. The thermal stresses imposed on the feedwater inlet nozzle and the surrounding wall portion of the steam generator vessel will reduce the life expectancy of this equipment, if the undesirable cooling is not eliminated. Therefore, repair of the thermal sleeve is required whenever such leaks occur. However, the repair of the thermal sleeve has proven to be a difficult task, because of the restricted access to the dissimilar metal weld which is used to secure the thermal sleeve to the feedwater inlet nozzle.
Accordingly, this prior art feedwater inlet nozzle, thermal sleeve and sparger assembly has encountered limitations as to, the operating conditions of the feedwater system with respect to reactor start-ups and changes in reactor power output, and also with respect to feedwater flow-induced vibration and fretting of the thermal sleeve, and further with respect to the repair of the thermal sleeve. Thus, there is a need to provide industry with solutions to these problems.
These difficulties are overcome, to a large extent, through the practice of the present invention which provides an improved apparatus for supplying feedwater to a nuclear type steam generator pressure vessel. The apparatus is generally comprised of a feedwater inlet nozzle, a thermal sleeve and a sparger, and is structured to supply relatively cool feedwater as compared to its heated self and the heated pressure vessel, while moderating the thermal gradients across the feedwater inlet nozzle and the surrounding wall portion of the pressure vessel; reducing the feedwater flow-induced vibration and fretting of the thermal sleeve; improving the structural support of the thermal sleeve and sparger; and facilitating the repair of the thermal sleeve.
Accordingly, there is provided a feedwater source including a conduit to supply the feedwater to the thermal sleeve which extends through the bore of the feedwater inlet nozzle and through an inlet in the steam pressure vessel wall. The thermal sleeve, which is fixedly supported by the feedwater nozzle, conveys the feedwater to the sparger located in the steam pressure vessel. The underside of the sparger includes a plurality spray holes which inject the feedwater downward into an annular passageway formed between the pressure vessel wall and a shroud that defines the evaporator chamber. The downstream end of the sparger is closed off by a generally flat plate which acts to deflect the feedwater toward the spray holes. The deflector plate can either be formed as an integral part of the sparger or be welded thereto. The deflector plate is advantageously sloped at an angle of 45 degrees measured clockwise from the longitudinal axis of the sparger so as to smoothen the flow of feedwater through the thermal sleeve and the sparger, thereby lengthening the life expectancy of the apparatus by reducing the flow-induced vibration and fretting.
The feedwater nozzle has its inlet face welded to the discharge end of the feedwater supply conduit, and also to the thermal sleeve as one of the two points used to support the sleeve. The other of the two points used to support the thermal sleeve is a weld between the outlet end of the feedwater nozzle and the thermal sleeve. This two-point support arrangement acts to increase the mechanical strength of the feedwater apparatus and, particularly, that of the thermal sleeve and sparger assembly, with a concomitant reduction in stress corrosion. The welds providing the two-point support for the thermal sleeve and sparger assembly are dissimilar welds to accommodate cost restraints requiring that the feedwater nozzle be made out of a metal composition that is less resistant to corrosion than that used in the making of the thermal sleeve. As a result, the feedwater nozzle side of the dissimilar weld will eventually become severely thinned and require repair. The feedwater apparatus is advantageously structured in that all of the welds, including the two dissimilar welds used to fixedly attach the thermal sleeve to the feedwater nozzle are readily accessible for inspection and repair.
The feedwater inlet nozzle has a cylindrically-shaped inner surface which defines a bore extending therethrough. The feedwater nozzle has an inlet and an outlet end portion wherein the bore is sized to obtain a tight fit or, alternatively, an interference fit between the inner surface of these nozzle portions and the outer surface of the correspondingly adjacent portions of the thermal sleeve. The feedwater nozzle inner surface which lies intermediate of the tight-fitting nozzle end portions is configured to form a recess therein and to cooperate with the recessed walls and the outer surface of the thermal sleeve to define an annular chamber therebetween. The chamber is provided with one or more threaded passageway openings extending through the body of the feedwater nozzle. A threaded plug is also provided to shut off the passageway opening. The chamber extends over a major length of the feedwater nozzle bore and is filled with a dry gaseous medium, for example, dry nitrogen or dry air, thereby forming a thermal barrier between the relatively cool feedwater flowing through the thermal sleeve and the heated surrounding portions of the feedwater nozzle and pressure vessel wall, and thus moderating the thermal gradients and the thermal stresses resulting therefrom. The use of dry nitrogen gas is preferred since it reduces stress erosion in the chamber.
A collar is coaxially disposed around the feedwater inlet nozzle intermediate the inlet and outlet portions thereof. The collar is normally formed as an integral part of the feedwater nozzle, and has a downstream end portion welded to the pressure vessel wall and an upstream end portion abutting a flanged ring which is provided with a plurality of circumferentially spaced apertures. The pressure vessel wall includes a plurality of apertures circumferentially spaced around the vessel wall inlet and penetrating the wall. These apertures correspond in number and arrangement to the apertures provided in the flanged ring. Fastening means that are generally in the form of threaded studs and lock nuts are used to clamp the flanged ring against the collar so as to forcibly and further secure the feedwater inlet nozzle to the pressure vessel wall. The collar includes an annular portion which is located intermediate of the downstream and upstream end portions of the collar. The annular portion of the collar is advantageously configured with a plurality of circumferentially spaced grooves that serve to lengthen the path of heat conduction, and thereby reduce the thermal gradients and the thermal stresses resulting therefrom. The land segments formed between the grooves provide the force transfer path used to rigidly secure feedwater inlet nozzle to the pressure vessel wall.